The present invention is directed to a method and apparatus for making color prints and more particularly for making half-tone screens for use in color printing.
In the conventional half-tone color reproduction process, an original color print or transparency is scanned by a photosensitive device that senses the variations in light intensity at each of the three primary color frequencies, namely, red, blue, and green. The average light intensity level for each primary color for each incremental area (pixel) of the scanned original is quantized and stored digitally. Alternatively, the digital values for red, green, and blue could be from other sources, such as by a programmed computer. This data is then processed to convert the values to the equivalent light intensity levels required to produce the same color from the three primary pigment colors, cyan, magenta, and yellow. It is also desirable to introduce a "black" component in addition to the three basic colors in the processed output data.
A set of four screens are produced, using a suitable plotter, from these four sets of values. Each screen, called a half-tone screen, is in the form of a grid. Depending on the printing process, the grid may be in the form of physical openings or in the form of clear areas of a photographic negative. In the half-tone process, the ratio of the area of each opening or clear area to the surrounding opaque area is determined by the required color intensity for the particular pixel of the reproduced image. Each screen is then used to lay down a grid pattern of dots of the associated one of the three primary colors on the color print reproduction. The resulting color print is a reproduction of the original but composed of certain patterns of four dots of varying size. The human eye integrates these dot patterns into the various color tones and detail of the original.
One problem with the superposition of multiple grids is the resulting formation of interference bands or patterns, known as the Moire effect. This effect is present whenever sets of parallel lines are superimposed at relative angles to each other. Where the sets of lines cross, they reinforce each other, producing dark bands in the picture. To minimize this effect in the color printing half-tone process, one technique is to place the superimposed grids at precise angles relative to each other, namely, 0 degrees for yellow, +15 degrees for cyan, -15 degrees for magenta, and +45 degrees for black. This solution and ways of implementing grid angle control are discussed in U.S. Pat. Nos. 4,456,924 and 4,499,489. One problem with this technique is that a small deviation from these precise angles produces a noticeable Moire effect. The Moire effect is also minimized if screens are aligned at the same angle. However, this approach has not been considered practical since, when using the same screen geometry, any slight variation in lateral displacement changes the amount of overlap of the dots and this in turn changes the ratio of white area to color area, resulting in noticeable changes in color reproduction.